Mobile Hub: Add possition field in API and allow frontend value setting

### Summary
Update the **Featured** API response for the Discover so elements have `position` value and the payload is already sorted according to that value for the client to consume.

This requires backend changes to return ordered carousel elements (with an integer position field), and frontend tooling in **Mobile Hub** to allow admins to edit carousel positions via a web interface.

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## Backend Requirements (API)

- [ ] Modify the carousel API response to include a new field: `position`
- [ ] `position` must be an **integer** (1, 2, 3, ...)
- [ ] The API response **must return items already sorted by `position` ascending
- [ ] Prevent duplicate positions

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## Frontend Requirements (Mobile Hub)

UI support in [Mobile Hub ](https://decentraland.org/mobile-hub)to to set `possition `value:

### Admin Capabilities
- [ ] View current featured carousel items
- [ ] Edit `position` value or each Featured item
- [ ] Save updated ordering (persist to backend)

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